Importance of the emitter in thin back-surface field solar cells

Abstract

The variation in the short-circuit current density J sc, the open-circuit voltage V oc and the efficiency η with the base thickness of an n +-p-p + back-surface field (BSF) solar cell with a resistivity of 10 Ω cm is reported for 1 sun air mass 0 illumination. It was found that when the cell thickness is small the cell characteristics are determined by the properties of the emitter. The effects of different emitter dark saturation current densities J E0 and different base diffusion lengths L B on the variation in V oc with the change in base thickness were calculated. If the base diffusion length is taken to be equal to or greater than 1000 μm for J E0 = 2 × 10 −12 A cm −2, the calculated variation in V oc with the base thickness is found to be in good agreement with the observed result that V oc is independent of the cell thickness in the base thickness range 100–300 μm. The calculations which were made for the case when a light reflector is present at the back of the BSF cell show that the maximum efficiency, which is obtained at a base thickness w B ≈ 200 μm, is appreciably increased. The effective surface recombination velocity S eff at the edge of the low-high junction is calculated as a function of the thickness w H of the high region for various values of the recombination velocity S r at the back contact. The calculations show that for S r > S v (where S v is the diffusion velocity in the high region and is equal to D H/ L H, and D H and L H are the diffusion coefficient and the diffusion length of the high region respectively) small values of S eff are obtained when the high thickness w H is greater than the high region diffusion length L H ( w H ≈ 3 L H). For S r = S v, S eff is independent of the thickness of the high region and has a value given by S v N B/ N H; when S r < S v, the high region must be thin but the thickness must be larger than the depletion width of the low-high junction. The temperature degradation coefficient of V oc for a BSF cell was calculated with the band gap narrowing in both the emitter and the high region taken into account. The band gap narrowing in both these regions reduces the temperature degradation coefficient of V oc.